The invention relates to a system and to a method for suppressing a periodically occurring interference signal during a bidirectional data transmission of data symbols between two xDSL transceivers.
The generic term xDSL combines in it a multiplicity of transmission systems for the twin copper wire of the telephone subscriber line network. The abbreviation DSL (digital subscriber line) indicates that information is transmitted in digital form. The most well-known xDSL technologies are ADSL (asymmetrical digital subscriber line), HDSL (high-bit rate digital subscriber line) and VDSL (very high-bit rate digital subscriber line).
FIG. 1 shows the bidirectional data transmission between two conventional xDSL transceivers via a data transmission medium. In DSL, data are transmitted in full-duplex mode which can be implemented without mutual influence as a supplement to previous telephone signal transmission. In DSL, data are transmitted in a first data transmission channel, the forward channel, from a network node to a subscriber line and, conversely, data are transmitted from the subscriber line to the network node in a reverse channel. Various variants can be distinguished in dependence on the bit rate of the forward channel. If forward and reverse channel have the same bit rate, this covers SDSL (symmetrical DSL). Since DSL is primarily designed for demand services, a lower bit rate is normally used for the reverse channel than for the forward channel. For this reason, the forward channel has a higher bit rate than the bit rate in the reverse channel in an ADSL system. VDSL systems mainly bridge the data transmission link between cable branches and customers whereas HDSL and ADSL transmit data mainly from the exchange to the customer or subscriber.
The standardized transmission method for transmitting data with ADSL and VDSL2 is the multi-carrier method DMT (discrete multi-tone transmission). In this method, the frequency band is divided into a number of bands, the lowest frequency band being provided for transmitting the conventional telephone signal POTS (plain old telephone service). This is followed by the frequency bands for the uplink or reverse channel and the downlink or forward channel. In DMT, the frequency bands are subdivided into up to 4095 subchannels, each subchannel having a frequency bandwidth of 4.3125 kHz. In each subchannel, modulation is performed by means of QAM (quadrature amplitude modulation).
FIG. 2 shows a conventional xDSL transceiver.
The conventional xDSL transceiver has a transmit signal path and a receive signal path. In the transmit signal, a data source delivers a data signal to a scrambler which scrambles the data. The scrambler eliminates long sequences of zeros or ones. During the scrambling, the original order of the data bit stream is altered in accordance with a selected algorithm. Long sequences of zeros or ones are converted in such a manner that frequent signal changes occur. Furthermore, the transmit signal contains a forward error correction unit FEC which, for example, performs Reed-Solomon coding. Reed-Solomon coding enables data transmitted with errors to be corrected. In particular, the Reed-Solomon codes allow error bursts to be corrected such as occur, for example, with DMT. Reed-Solomon codes provide for so-called forward error correction, i.e. error correction does not require a reverse channel. Test numbers are calculated which are appended to a data block to be protected and are transmitted together with this block. The Reed-Solomon code word transmitted therefore consists of useful data and test data.
The data bits to be transmitted are interleaved in the time domain by an interleaving unit. The interleaver distributes the code words of the Reed-Solomon encoder over a greater range of time so that any transmission disturbances which may occur are distributed over a number of code words.
A trellis encoder inserts further redundancy into the data stream. The additional bits are then used in the receiver for error correction. Whereas the Reed-Solomon encoder operates by block, i.e. adds a block of test bites to a defined data block, and thus performs block encoding, the trellis coder performs convolution coding. The data to be protected are continuously linked to a protection polynomial so that redundancy bits are permanently inserted into the data stream.
A block of data bits to be transmitted which is delivered by the trellis encoder is temporarily stored in a data buffer with the transmit signal. In a QAM encoder, a QAM symbol is generated for each carrier of the DMT data transmission system, i.e. a pointer is generated in the constellation diagram or, respectively, a complex number is generated. A QAM encoder produces the spectral lines of the signal to be transmitted with complex numerical values. The spectrum thus generated is transformed into a time domain signal with an IFFT (inverse Fourier transformation from the frequency domain to the time domain) unit. The samples of the time domain signal thus produced are successively interpolated by an interpolation filter IF and converted into analogue signals and are sent out after low-pass filtering.
Conversely, a low-pass filtered time domain signal is sampled on the receiving end of the xDSL transceiver and read into a signal buffer after decimation by means of a decimation filter. The low-pass filter reduces aliasing effects so that spectral components outside the frequency band used do not noticeably corrupt the sampled signal. One data block of N samples is in each case transformed into the frequency domain by means of FFT (fast Fourier transformation). Each spectral line of the discrete spectrum thus calculated represents a QAM data symbol from which a bit combination is then obtained. The operations performed in the transmit signal path are performed in the reverse order in the receive signal path. The receive signal path, therefore, contains a frequency equalizer, a trellis decoder, a deinterleaving unit, a decoder and a descrambler.
The transmit signal path and the receive signal path are connected to the two-wire telephone line via a hybrid. Signal components of the signal sent out by the transmitter of the xDSL transceiver are reflected on the transmission link and form echo signals. These echo signals pass via the hybrid into the receive signal path where they lead to disturbances. The xDSL transceiver, therefore, contains an echo compensation filter EC which calculates from the transmitted signal the echo signal to be expected, the calculated echo signal then being subtracted from the receive signal by means of a substractor. The form and the duration of the echo signal depend on the configuration of the subscriber line. The echo compensation filter EC is therefore preferably constructed to be adapted and can then be adapted to the respective echo signal characteristic to be expected.
The conventional xDSL transceiver also contains in the receive signal path an equalizer EQ for compensating for linear distortions in the receive signal in the time domain.
During the data transmission between two conventional xDSL transceivers via two-wire telephone lines within a bundled cable, electromagnetic disturbances which can lead to bit errors during the data transmission occur due to electrical coupling from other systems which transmit data in the same bundled cable or from other systems such as, for example, radio or TV transmitters or other electrical devices. The Reed-Solomon coders provided in ADSL and VDSL transceivers insert redundancy into the data stream in order to be able to correct a particular density of errors. The large proportion of the interference signals coupled into the two-wire telephone line consists of periodically occurring interference pulses which are generated, for example, by switching processes in devices. These interference signals are coupled into the telephone lines which are in the vicinity at the subscriber. Typical devices which generate periodic interference signals are dimmers, neon tubes and switched-mode power supplies.
Even short interference pulses can corrupt or destroy a complete DMT data symbol so that a large number of data bit errors are produced. Periodically occurring interference signals are also called REIN (repetitive electrical impulse noise) signals.
The methods for suppressing interference signals hitherto used do not utilize the periodicity of the interference pulses for reducing the expenditure in order to minimize or prevent transmission errors. The coding methods used in xDSL transceivers such as, for example, FEC coding methods, do not locate the interference signal but only insert sufficient redundancy for error correction into the bit stream. During the redundant coding, the interleaver distributes the disturbed data over as many code words as possible since each code word only has a limited possibility for error correction. Adding redundancy and interleaving by means of interleavers increases the run time during the data transmission.
To minimize the run times and to minimize the technical expenditure or complexity during the coding, a method for suppressing interference signals during ADSL and VDSL data transmission has been proposed, therefore, in 2Wire, “Periodic Impulse Noise: How predictable is it?” ITU SG15/Q4 contribution D-035, Geneva, Switzerland, November 2004, in 2Wire, “Multi-Rate Impulse Protection”, ITU SG15/Q4 contribution HH-081, Waikiki, Hawaii, January 2005, 2Wire, “When to incorporate frame-blanking in VDSL2”, ITU SG15/Q4 contribution HA-094, Huntsville, Ala., March 2005 and in 2Wire, “Frame-Blanking: A Simple and Effective Method of REIN Protection”, ITU SG15/Q4 contribution HA-093, Huntsville, Ala., March 2005, in which certain data transmission symbols are not used for data transmission if the occurrence of an interference pulse is expected.
FIG. 3 shows a conventional data transmission system for transmitting data between two xDSL transceivers, in which, after echo compensation and equalization in the transceiver B, a signal is picked up for interference signal detection. When the periodic occurrence of an interference pulse or of an interference signal is detected, its position and period is reported to the transceiver A at the other end of the DSL line via an overhead channel. From this, the transceiver A internally generates a synchronization signal and periodically delivers a disable control signal to the IFFT unit on its transmit signal path. Instead of the transmit data symbol which is presumably corrupted during the data transmission via the data transmission channel due to the interference signal, either no data symbol, a permanently defined data symbol or a randomly generated data symbol is transmitted. The random data symbol contains random values. The data symbol does not contain any useful data in any of the cases mentioned.
A disadvantage of this known method for suppressing interference signals according to the prior art consists in that, in the case where an interference pulse lies on the boundary between two successive data symbols or overlaps the two data symbols, neither one of the successive data symbols can be used any longer for useful data even if the interference pulse is shorter than a data symbol.
In Europe, the mains frequency ƒN is 50 Hz and in the US is 60 Hz. A dimmer which switches at a frequency ƒN of 50 Hz and causes an interference signal with each rising switching edge and each falling switching edge generates an interference signal with an interference frequency ƒS of 100 Hz in Europe and 120 Hz in the US. On the data transmission line, therefore, an interference pulse, which is caused, for example, by a dimmer, occurs every 10 ms (in Europe). If the data symbol length TD, for example in ADSL, has a duration of 250 μs, an interference pulse occurs every 40 data symbols. If the interference pulse is located on the boundary of two data symbols or overlaps two data symbols, two data symbols are not transmitted in a conventional method. Thus, two of 40 data symbols are not transmitted. This corresponds to a data loss of 5%.